70 GEOLOGY. 



morphic, at other times unchanged ; sometimes it is quartzose, and imperfectly mixed with 

 large scales of silvery mica, in other places feldspar prevails. 



This sierra is scarcely more than one mile wide where the line crosses it ; both sides are bor- 

 dered by the upheaved and contorted crystalline beds just alluded to. We ascended to the top 

 of this sierra, near where the singular-looking peak that marks the Escondida towers up, and 

 found it to exceed in barrenness either of the sierras — Pajarito, Janos, or Atascosa. Portions of 

 the terrace-like slopes, and also the plateau, are covered with patches of white or pearl-colored 

 chalcedony, investing the rocks with a scoria-like crust of that silicious fossil. The southern 

 part exhibits a more horizontal arrangement, leading to the supposition of having been formed 

 under water ; for here are extensive table-lands, ridges, lomas, and mesas, composed partly of 

 black vesicular or compact trap, and partly of real quaternary banks. The topography of the 

 country seems to indicate here the confluence of numerous mountain streams and torrents 

 coming from every direction. (See sketch No. 40 of azimuth line.) 



The line crosses a little to the north of a conspicuous peak(D) — the highest point of the whole 

 range — and falling on the Mexican side, we gave it the name of " Cerro de Sonora." 



Immediately west of the Sierra de la Escondido a low group of granite hills(c) occur, furnishing 

 several temporary, as well as permanent, water places, apparently well known to the natives — 

 Papagos and Apaches. Some are mere tinajas ; others real springs, though liable to become 

 dry before the setting in of the rainy season. While encamped here we experienced a heavy 

 hail and thunder storm ; in a few minutes water came rushing down the ravines in a torrent, 

 five feet deep, carrying everything before it, and giving us unmistakable proof how little time 

 it requires to submerge all the valleys around under a most terrible flood of rain-water. This 

 mountain group was called " G-ranizo," (hail,) and is so designated in the maps from the cir- 

 cumstance of the surveying parties being overtaken here by one of those hurricanes peculiar to 

 these regions. 



A flat valley, nine miles wide, separates the Sierra de la Escondida and the adjoining Granizo 

 group from the Sierra Verde, which is a southern spur or branch of the Sierra del Babuquibari, 

 north of the line. The plateaus bordering the dry water-courses of this valley furnish fine 

 grass, and are sparsely covered with well-developed hackberry and liveoak. The Sierra Verde, 

 so called because of the verdure encountered in the shelter of its rocky valleys, seems to be formed 

 exclusively of feldspathic granite, similar to that already mentioned as occurring on the east 

 slope of the Sierra Escondida. The strike-side faces southwest, and with a width of 

 scarcely more than a mile, this sierra does not piesent any petrographic novelties. Its longi- 

 tudinal axis ranges southeast and northwest, and joins the bold walls of igneous rocks belonging 

 to the Sierra Babuquibari. At its southern end mounds of dark, vesicular trap crop out of the 

 diluvial main. Here water finds its way to the surface, forming a spring known as the " Pozo 

 Verde, " (Green Well ;) the bunches of rush, which at once conceal and mark the water, in all 

 probability gave rise to the name. 



Almost due north of the Sierra Verde lies thepicacho of the Sierra del Babuquibari, which is 

 one of the orographical phenomena of the country, its peculiarity being such as to attract espe- 

 cially the attention of the red man. The Papagos consider this huge mountain obelisk their 

 palladium ; here they take refuge in times of famine, drought, or war. Babuquibari is said to 

 signify " water on the mountain." The word is certainly formed from babu (water) and ari 



